The field of the invention is plastic forming apparatus, and the invention relates more particularly to the forming method known as blow molding and even more particularly to the operation relating to blow molding known as spin trimming. This process provides a highly efficient and effective method for removing the unwanted flashing from the top of hollow blow-molded objects such as bottles.
An early spin trimming apparatus is shown in U.S. Pat. No. 3,429,211 entitled "Apparatus For Trimming The Neck Of Hollow Plastic Articles". This patent discloses a device which grasps a bottle below the neck by a groove formed in the neck. A knife edge is held against a notch formed above the neck. The bottle is turned by being held against the knife edge and a protrusion which fits the groove in the neck while at the same time the groove is contacted by a moving belt on the other side and pushed against the knife in a turning action. The knife is positioned on an angle so that as the bottle turns against it, the knife cuts further into the neck.
A more elaborate device is shown in U.S. Pat. No. 3,675,521 entitled "Article Trimmer". This device holds the bottle from above the neck and also holds the bottle around the wall thereof.
U.S. Pat. No. 4,305,904 discloses another spin trimmer which supports the bottle from above the neck and moves it along the cutting knife held parallel to and below the moving belt. While spin trimmers find wide application in the finishing of blow molded objects, this process has several shortcomings. For many types of polymers, the knife blade, instead of cutting through the polymer wall, tends to collapse and deform the wall and the process is inoperative. This is particularly true for polymers such as polyvinyl chloride bottles and polyethylene terephthalate bottles which are widely used for soft drinks. Furthermore, many of the prior art spin trimmers are deficient in holding the bottle steady in a verticle position and instead tend to wobble as they move along the device. This leads to an uneven neck opening.